The Sentinelese have also never been truly isolated. They are just too close geographically to other people. We know that Sentinelese words occur in other Andamanese langauges and we know from British records in the 19th century that there were Sentinelese actually living amongst the other Andamanese peoples. So long as one such event occurred every couple of generations, the Sentinelese are going to have the same ancestor as everyone else.

But it didn't. You have no evidence that it did. You're just asserting that it must have.

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It seems likely that the last common ancestor of Hawaiians and other Polynesians is less than 1, 500 years back. That would makes the Hawaiians less isolated than Europeans.

Hawai'ians less isolated from [other] Polynesians than Europeans from what? Not other Europeans.

That said, yes, it's fair to say that the OP is at most a 10th cousin (if I were betting in a pool, let's say 8th or less) of his girlfriend, unless their heritages are from other sides of the planet or something.